1. Field of the Invention
The present invention relates to a device for indicating a mark in an optical system such as view-finder of a camera, and more particularly such device having relief hologram to form a reconstructed image of the hologram on an image plane of the optical system.
The term "hologram" herein shall mean any hologram capable of providing a reconstructed image when illuminated and shall include those prepared by holographic interference of light, by computer calculation or by diffraction lattice, or copies thereof and the like.
2. Description of the Prior Art
A holographic indicator similar to the present invention is already disclosed in the U.S. Pat. No. 3,633,988, in which a hologram is disposed at a sight window provided in a helmet for a pilot to enable him to observe a distant object simultaneously with an image reconstructed from the hologram at a distant position.
Also the U.S. Pat. No. 3,560,921 discloses a device which permits at the same time the observing of an instrument and an indicating mark reconstructed from a hologram.
The present invention is directed to a device capable of forming an image of the object and a reconstructed image of hologram on a substantially same plane for simultaneous observation through a single eyepiece.
Also in a camera, the view-finder is required to provide a clear view of the object. However, the presence of a hologram in the light path of the view-finder will darken or cloud the image of object observable therein, which is generally referred to as darkening phenomenon. The camera proposed by the present invention is featured in that the image of object is not affected by the hologram disposed in the light path of the view-finder when the indicating mark is not reconstructed therefrom, thereby allowing clear observation of the image of object without such darkening phenomenon.
The U.S. Pat. No. 4,165,930 of the present inventors discloses a holographic indicator utilizing a volume hologram to achieve the aforementioned objects. Also the U.S. Pat. No. 4,255,032, corresponding to the Japanese Patent Laid-Open Sho55-18664, of the present inventors discloses a holographic indicator utilizing a relief hologram which is cheaper in cost and more suitable for mass production than the volume hologram. However, in order that the image of object and the reconstructed mark can be satisfactorily observed in the view-finder in the use of said holographic indicator in a camera, the grating pitch of the relief hologram has to be made small to avoid the entry of undesirable diffracted light therefrom into the light path of the view-finder as already explained in the above-mentioned patent. The preparation of such relief hologram becomes more difficult as the grating pitch becomes smaller.